Two primary methods of generator synchronization exist: manual and automatic. In manual synchronization, the operator adjusts the generator's speed and voltage and executes the breaker closure command. This process, in its most fundamental form, is carried out by the operator independently, without any external assistance or limitations.
The tecnique of synchronizing a generator to the grid involves adjusting the generator's speed and voltage.
In the above photo: Original gauges from the Pelzer Upper Hydro, circa 1920. The topmost gauge was used to synchronize each generator's output with the others as they were brought online. Despite the risk of significant damage if not properly synchronized, this operation was manually performed. The middle gauges display the voltage output of each generator. Uniquely, this facility was the only one in the world to generate power at 3,300 volts, excluding the original hydro operation. The lower left gauge shows the alternating current frequency at 60 hertz, while the lower right gauge indicates the power factor, a measure of efficiency.
To prevent out-of-phase closures resulting from human error, preventive functions are now employed to oversee manual synchronization. Over the past twenty years, both automated and manual synchronization methods have evolved to include intricate protective functions, which users can adjust.
For the initial sixty years, the power industry depended on the expertise of qualified operators for maintaining synchronization. The risk of a catastrophic out-of-phase closure was too great to entrust to an automatic system that might fail. As generators have increased in size and designs have advanced in efficiency, electrical and mechanical systems have become less forgiving of the imprecise synchronization by impatient or inattentive operators.
Manufacturers have shown less flexibility in their designs, as demonstrated by the stringent constraints on closing angle, voltage difference, and slip frequency.
The complexity of plant operations also saw a significant rise, leading to an increased workload for the operating staff and diverting their attention from the previously sacrosanct process of synchronization. Consequently, due to these changes and the severe damage resulting from certain operator errors, hands-off, fully automated synchronization methods have now become standard practice for equipment synchronization.
Synchronization equipment in plants may be operated either manually or automatically. The objective is to primarily utilize the automatic system, turning to manual operation only when required. Nevertheless, the chosen method often reflects the plant's operational philosophy and, occasionally, the degree of dissatisfaction with the automatic synchronization equipment.
1. Manual Synchronizing
Synchronizing equipment has significantly evolved since the early days of parallel generator operation, where a black lamp synchronizer was used. In this method, lights were connected across the open breaker's corresponding phases. Separate voltmeters were employed to measure the system voltage and the generator voltage.
Initially, the governor controls are utilized to bring the generator up to its rated speed. Subsequently, the voltage regulator is adjusted to align the generator's output with the electrical grid's voltage. The operator then monitors the lights, which flicker at the slip frequency, indicating the generator voltage's rotation relative to the system voltage.
If the generator and the system were 180 degrees out of phase, the lamp would shine at its brightest. Conversely, if the two voltages were perfectly in phase, the lamp would turn off. As the operator adjusts the generator speed, the light pulsations would diminish progressively.
This indicates a very close frequency match between the system and the generator. As the lights went out, the operator recognized that the voltages were synchronized and the phases aligned, prompting them to close the circuit breakers.
Clearly, the system is not flawless. The frequency difference upon closure relies on the operator's discretion or, occasionally, their impatience to set the maximum pulsation rate. The lights do turn off, but not at zero voltage, as a certain minimum voltage is necessary for them to emit light. Moreover, there is a delay between issuing the close signal and the actual disconnection of the breaker. The dark-lamp synchronization method offers a straightforward and economical solution.
Nevertheless, the development of more secure synchronization techniques has been prompted by the potential harm from significant out-of-phase closures and the reduced service life resulting from frequent severe closures.
The current manual synchronizing tools are depicted in Figure 1. It remains essential to measure the voltage on both sides of the synchronizing breaker to verify that the system and generator are functioning at the same voltage level. As illustrated in Figure 2, a synchroscope is utilized to visually confirm the frequency and phase angle alignment between the two systems.
The frequency indicator on the scope will rotate clockwise when the generator's frequency exceeds that of the system. Conversely, it will rotate counterclockwise when the generator's frequency is below the power grid's frequency.
Figure 1 – Manual Synchronizing
Rotational velocity signifies the frequency discrepancy between two systems. By adjusting the oscilloscope's position, the phase difference between the two voltages can be observed in real-time. When the clock indicates 12 o'clock, the voltages are synchronized. At the 2:00 position, the phase shift between the two sources is 60 degrees, equivalent to 36×2/12.
Consider a generator manufacturer that stipulates synchronization must happen within 10 degrees of the in-phase position and with a slip of less than 0.067 hertz to understand the process. The scope's rotation cannot exceed approximately one rotation every 15 seconds due to this slip restriction (one rotation in 14.9 seconds; 1 divided by 0.067 seconds).
The operator should close the circuit breakers when the synchroscope indicator reaches one-third of the distance from 12:00 to 11:00, adhering to the 10° angular limit (calculated as 10/360 × 12 = 0.33).
Figure 2 – Synchroscope
In a fully manual synchronization process, the operator issues an unattended close command to the breaker via the breaker control switch. Such operator-exclusive setups are nearly extinct.
A sync-check relay is now installed in series with the control switch to oversee the closure process when manual mode is activated. This relay monitors the phase angle between the generator and the system voltage. In generator applications, the voltages should be within a specified angular range (typically within 10° of the in-phase position) and the slip should also be within a defined limit for the relay to close its contacts.
Transmission lines require significantly larger angular settings. While the operator retains some control over the closing process, the capacity to execute a closure that is substantially out of phase has been restricted.
2. Automatic Synchronizing
Figure 3 illustrates that automatic synchronizers can close the breaker and synchronize the generator without human intervention. The operator has control over the generator's initial startup and acceleration. As the generator's speed increases, so does the voltage.
The automatic synchronizer can measure and synchronize the generator frequency within 70% to 80% of the rated voltage.
Once the slip, voltage magnitude, and phase angle thresholds are met, the synchronizer automatically activates the governor and voltage regulator. It then sends a signal to the synchronizing breaker to close, provided the operational parameters fall within the acceptable range.
A breaker closure signal can be produced by the type of synchronizer used, once all parameters are within the established limits. Upon meeting these limitations, the anticipatory synchronizer utilizes real-time slip data and the breaker closing time to calculate the necessary close initiation angle to achieve closure at the zero-degree position.
Figure 3 – Automatic Synchronizing
The synchronizer issues the closure command at the predetermined angle. Anticipatory synchronizers necessitate a degree of system slip to operate effectively. Contemporary synchronizers are capable of functioning with slips as minimal as 0.0001 Hz, equating to a single revolution of the synchroscope every 2.8 hours.
Achieving such precision in speed matching is rare. While a precise match is perfect for seamless synchronization, closing the slip breaker will be postponed by approximately 5 minutes for every 10 degrees that the generator voltage must traverse to attain the in-phase position at the slip breaker. When the voltage falls within permissible closing ranges yet the slip remains minimal, many anticipatory synchronizers will dispatch a bump pulse to the governor.
This function tolerates a slight increase in slip to expedite the breaker's shutdown. Automatic synchronizers provide various adjustable closing limit parameters to guarantee safe synchronization. Nevertheless, should the synchronizer malfunction, these limits become irrelevant.
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